This invention relates to light projectors and particularly to spotlights of the theatrical type for projecting an adjustable high intensity beam of light and is particularly adapted for use in theaters, auditoriums, arenas and the like where a spotlight more compact than the conventionally available spotlight is desired.
In the prior art it is conventional to have a spotlight consisting of a brilliant source of light placed in front of a concave reflector which gathers some of the emitted light from the source and directs it toward an aperture usually controlled by a shutter and an iris. Light emitted from the source and reflected from the reflector travels through the opened shutter and iris to a condensing lens adjacent to the iris and then to a front lens system which projects the light in a beam toward the desired object. Newer spotlights of this type have been developed utilizing spaced electrical discharge electrodes contained in part in a sealed gas filled envelope. When operated by applying the appropriate electrical power, a brilliant light source is emitted from the space between the two opposed electrodes, the space containing an arc flame. Conventionally two methods have been utilized for positioning such a source with respect to the reflector.
In a first method typified by U.S. Pat. No. 3,173,617 the source is mounted such that the two opposed electrodes form a line perpendicular to the optical axis of the projector and lie in a plane somewhat in front of the plane defined by the leading edge of the reflector. Assuming the reflector to be ideal, that is no energy lost upon reflection, the amount of light directed by the reflector toward the aperture of the apparatus is considerably less than 50 percent of the emitted light since over half the emitted light is emitted in the forward direction thus never reaching the reflector.
A second method consists of mounting the source such that the electrodes of the source are conincident with the optical axis of the system as shown in U.S. Pat. No. 3,624,386 and U.S. Pat. No. 3,702,395. While increased optical efficiency can be experienced since a greater portion of the emitted light from the source is gathered by the reflector and is directed toward the aperture system, it is necessary that the system be designed such that the front terminal of the forward directed electrode intersect as little light as possible since the light intersecting the electrode will heat the electrode. If the amount of light intersected by the front electrode becomes too great, the end of the lamp will suffer damage and will become inoperative.
It has been the practice in the past to choose a reflector having a diameter substantially larger than the length of the source and having a focal length two to three times the length of the source. Central sections of the reflector have been eliminated to insure that little or no radiation intersects with the front terminal of the axially mounted source. The long focal length requirement of such a reflector contributes to the overall size of the projector by requiring the distance between the aperture and the reflector be one and one-half to three times the length of the source.
Ellipsoidal reflectors have been used in combination with incandescent lamps where the lamp bulb was inclined with respect to the optical axis of the reflector much like U.S. Pat. No. 2,769,082. Since the incandescent bulb has no forwardly directed electrode which needs to be protected from heat, little appreciation of the problems involved has been found in the prior art of this type.